Two radically different methods are used nowadays for drilling wells. One method is a rotary drilling method, where in the drive of a rock-breaking-tool--bit--is disposed on the ground level, and the bit is rotated through a string of drill pipes. The second method involves the employment of downhole hydraulic machines disposed directly above the bit. The drill pipe string remains stationary in this case. The second method has a number of obvious advantages: there are no energy losses for rotating the drill pipe string; the load on the drill pipes is lowered, hence, the number of emergency situations in the well shaft is reduced.
Downhole screw motors are most widespread used among all types of downholes motors employed nowadays in practical drilling applications. These motors feature easy operation and maintenance; they are compact and make it possible to work with drilling muds of largely varying density and viscosity (cf. Gusman M. T., Baldenko D. F., et al. Downhole Screw Motor for Well Drilling. M., Nedra Publishing House, 1981). Such hydraulic motors generally comprise a casing, an output shaft having radial and thrust bearings, and working members consisting of two elements: an outer rubberized sleeve or stator having internal helical teeth and a rotor and shaft having outer helical teeth accommodated in the stator. The number of teeth of the sleeve is greater than the number of teeth of the shaft by unity so that the interior of the working members is divided into high and low pressure chambers by their mutual engagement when a liquid is pumped through the working members. Under the action of the resultant pressure difference, the rotor starts moving relative to the stator, the axis of the rotor describing a circle about the axis of the stator. This rotation is transmitted to the output shaft of the rotor. Usually a flow of liquid is used as a source of energy for operation of the motor, but the hydraulic motor can also function using an aerated liquid or compressed air.
Nowadays downhole motors used for drilling wells general have the whole flow of liquid passing between the rotor and stator.
One of the main disadvantages of such motors is the dependence of their output parameters on the inlet flow of working liquid. As the production requirements in drilling wells do not frequently take into account energy capabilities of downhole motors, the latter are often used under unfavourable conditions when speed and pressure differences are too high thus resulting in premature failure of parts and assemblies of the motor.
To eliminate this disadvantage, conoid nozzles are mounted in the axial passage of the rotor (cf. USSR Inventor's Certificate No. 436595, Cl. E 21.B 4/00, 1972), the flow of working liquid being throttled through the nozzles.
The characteristic of such motors is dropping, i.e. the output shaft speed decreases with a growth of the load torque much faster than in hydraulic motors that do not have such nozzles. As the load increases, this results in a strong speed decrease until the output motor shaft stops at low load torque values thus resulting in a lower output torque, hence, in a lower efficiency of drilling.